A hard fork initiation is a governance action that moves Cardano to a new major protocol version, introducing changes to the ledger rules that older node versions cannot follow. It is how protocol eras advance under on-chain governance instead of by developer decision alone.
All three governance bodies vote on it: SPOs, DReps, and the Constitutional Committee must each approve it before the upgrade proceeds. It is one of the few action types where the SPO threshold is binding alongside the DRep and committee thresholds, so a hard fork needs a Yes from every body. SPO approval matters practically as well as politically, since block producers must actually run the new node software for the fork to happen smoothly.